The fabrication of integrated circuits requires a method for accurately forming the circuit-defining image patterns on a semiconductor wafer. A photoengraving process know as photolithography, or simply masking, is widely employed for this purpose. The microelectronic circuit is built up layer by layer, each layer being based on a particular image pattern received from a photolithographic mask. Such masks typically comprise a glass plate approximately the size of a wafer, the plate having a single pattern repeated many times over its surface. Each repeated pattern corresponds to a pattern to be imposed upon a layer of a wafer.
The mask patterns are derived from an optical reticle having a primary pattern which may be generated by a computer controlled light spot or electron beam which is scanned across a photosensitive plate. In early systems involving masks, the reticle patterns were typically ten times the final size of the pattern to be imposed on the wafer. An image one-tenth the size of the reticle pattern was projected optically on the final mask, such reticle pattern being reproduced side by side many times on the mask in a step-and-repeat process. Thereafter, the mask patterns were transferred in a number of ways to wafers, for example, by optical projection scanners.
In later-developed unit magnification scanners systems, full size, multiple pattern masks have been eliminated to a great extent because it has become possible to repetitively align and focus on to a small wafer a reticle pattern the same scale as the final pattern. However, some problems remain in applying such systems.
U.S. Pat. No. 4,391,494 (Hershel), which issued on Jul. 5, 1983, discloses an apparatus which substantially improved upon the resolution of the above-mentioned projection scanners. The apparatus according to Hershel projected an image of a reticle pattern onto a wafer with one-to-one magnification.
A schematic representation of the Hershel apparatus is attached hereto as FIG. 1 which includes a mirror 10 and a composite achromat-prism assembly 12 which are disposed symmetrically about an optical axis 14. The reticle pattern plane 16 lies on one side of the axis 14 and the wafer image or object plane 18 lies on the opposite side. The prisms 20 and 22 couple light into and out of the optical system and separate the reticle plane 16 from the horizontal wafer plane 18. An air gap between the reticle plane 16 and the prism 20 and the wafer plane 18 and the prism 22 provide sufficient mechanical clearance and space for movement of a wafer and a reticle into and out of the respective wafer image plane 18 and reticle pattern plane 16. This system has proved quite advantageous and useful with systems of moderate to low numerical aperture (NA). However, because of the use of prisms 20 and 22 the system inherently includes a certain amount of field which is lost due to vignetting that is dependent on numerical aperture.
The system described in the Hershel patent is a unit magnification achromatic anastigmatic optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures. The system is basically symmetrical, thus eliminating all aberrations of odd order such as, distortion and lateral color. All of the spherical surfaces are nearly concentric with the centers of curvature located close to the focal plane. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary. However, in order to attain sufficient working space for movement of the reticle and wafer, the object and image planes of this system are separated through the use of two symmetrical folding prisms. The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field.
With the increasing demand for higher resolution capabilities from such systems, a system capable of even higher numerical apertures and higher resolution with acceptable field size was developed. This system is disclosed in U.S. Pat. No. 4,964,705 (Markle), which issued on Oct. 23, 1990. The system disclosed in the Markle patent utilizes nearly 50% of the total field to create an image on a wafer. By sacrificing the remaining 50% higher numerical apertures and higher resolutions can be obtained while maintaining acceptable field size.
The Markle unit magnification optical system is depicted in FIG. 2, attached hereto, wherein a source of exposure energy 40 generates a beam of energy which is directed through an aperture 42 to a relay 44 which, for example, may be comprised of a plano-concave element 46 and a plano-convex element 48. The converging illumination beam is then directed through a primary meniscus lens 50 having a partially reflective surface 52. Relay 44 and primary lens-mirror (50,52) are symmetrically disposed about an optical axis 54. The primary lens is preferably made of fused silica and includes a first surface 56 closest to relay 44 and the source of exposure energy 40 in addition to the partially reflective surface 52. That portion of beam 40 which passes through surface 52 is directed to a refractive lens assembly or group 58 for receiving and transmitting that portion of the beam of energy. The refractive lens assembly 58 preferably includes at least a meniscus lens 60 made from a material having a relatively high index of refraction, for example, fused silica or barium fluoride. The lens 60 has a first convex surface 62 facing the primary lens 50 and a concave surface 64 facing away from the primary lens 50. The refractive lens assembly 58 further includes a plano-convex lens 66 preferably made from a material having a lower index of refraction and a lower dispersive power than meniscus lens 60, for example, lithium fluoride and has a convex surface 68 facing the concave surface 64 and a flat or nearly flat surface 70 facing away from the primary lens-mirror (50, 52). A reticle element 80 is positioned in close proximity to the flat surface 70 of plano-convex lens 66. A wafer 90 is positioned parallel to and in close proximity to reticle element 80.
The aforementioned monocentric system exhibits aberrations due to spherical aberration and color of the chief ray. These monocentric afocal systems with the object and image at the common center of curvature have only the aberration of higher order astigmatism. That is corrected to one order higher than the departure of the chief ray from afocality. If the design is paraxially afocal (i.e., corrected for afocality to the 1st-order) then 3rd-order astigmatism and Petzval curvature are corrected. If it is only paraxially afocal for one wavelength because of longitudinal color of the chief ray, then the 3rd-order astigmatism and Petzval curvature will change with wavelength.
If the system is afocal at the 3rd-order level, as well as paraxially, due to 3rd-order spherical aberration of the chief ray being corrected, then 5th-order astigmatism and Petzval curvature are also corrected. If there is spherochromatism of the chief ray (i.e., variation of spherical aberration with wavelength), then 5th-order astigmatism and Petzval curvature will change with wavelength.
If the 5th-order spherical aberration of the chief ray is corrected, then 7th-order astigmatism and Petzval curvature are also corrected, and so on.
The designs in FIGS. 1 and 2 above correct the longitudinal color and the 3rd-order spherical aberration of the chief ray. This is accomplished by introducing a cemented monocentric surface with two different glasses on either side of the refractive lens assemblies. The dispersion difference between the two glasses corrects the longitudinal color of the chief ray, while the index difference corrects 3rd-order spherical aberration of the chief ray. As the index difference is made less, the required dispersion difference also becomes less. Due to their chief ray properties, these conventional designs can be corrected for chromatic variation in 3rd-order astigmatism and Petzval curvature as well as the monochromatic 5th-order aberrations.
In fact, this is almost never done since it is always better to balance out the uncorrectable higher order aberrations by introducing compensating amounts of the lower order aberrations which can be controlled. In this case, the longitudinal color of the chief ray in the conventional designs is only partially corrected, so as to balance some intentional residual color against the uncorrectable chief ray spherochromatism (i.e., chromatic variation of spherical aberration) of the design. In a like manner, 3rd-order spherical aberration of the chief ray is only partially corrected so as to balance some of it against the uncorrectable 5th-order spherical aberration of the chief ray.
The present inventors have developed a unique configuration of the refractive lens group or assembly of the unit magnification optical system which utilizes a gap, preferably an air gap, disposed between a meniscus lens and a closely spaced plano-convex lens of the lens group, thereby to improve both the monochromatic correction and the chromatic correction of the system. Such improvement is directed to a principal drawback of previously known arrangements. With two different lens materials, it is possible in a design with cemented lenses to correct either higher order astigmatisms or chromatic variation of astigmatism, but not both, unless the lens materials have a particular relationship to each other. Most lens materials cannot exactly satisfy that relationship, so there is a compromise made and performance suffers. With the addition of a small air gap to the design, many lens material pairs can be found with the right relationship, allowing correction of both higher order astigmatisms and chromatic variation of astigmatism.
The present invention involves several modifications to the refractive lens assembly which substantially improve the imaging performance of the unit magnification optical system. These modifications include the use of more than one meniscus air gap in the refractive lens assembly, which result in correction of higher order aberrations and enhance the performance of the system. For example, the present invention permits correction to all the monochromatic 3rd, 5th, 7th, 9th and 11th-order aberrations in field and aperture.
The present invention also provides many additional advantages which shall become apparent as described below.